1. Field of the Invention
The present invention relates to a sublimation-type thermal image transfer recording medium for use with copying machines and printers, and more particularly to a sublimation-type thermal image transfer recording medium for multiple printing recording by a thermal image transfer recording method under the condition that the transporting speed of an image receiving sheet is larger than that of the sublimation-type thermal image transfer recording medium (hereinafter referred to as n-times-speed mode method) when images are printed.
2. Discussion of Background
Recently the demand for full color printers is increasing year by year. Representative examples of recording methods for full color printers now available include the electrophotographic method, the ink-jet method, and the thermosensitive image transfer recording method. Of these methods, the thermosensitive image transfer recording method is most widely employed because of its advantages over the other methods in that the maintenance is easy and the operation is noiseless.
In the thermosensitive image transfer recording method, a thermal image transfer recording medium, which is a so-called color ink sheet, and an image receiving sheet are employed. The thermal image transfer recording medium comprises a support, and an ink layer formed thereon, comprising a color ink is composed of a coloring agent dispersed in a thermofusible material, or a sublimable dye dispersed in a binder agent.
To carry out the thermosensitive image transfer recording, the image receiving sheet is superimposed on the ink layer of the thermal image transfer recording medium and the color ink is transferred imagewise from the thermal image transfer recording medium to the image receiving sheet by thermally fusing of the coloring agent or the sublimation of the sublimable dye under the application of thermal energy to the recording medium by laser beams or through a thermal head which is energized by the electric signals corresponding to the images to be recorded.
The thermosensitive image transfer recording methods can be roughly classified into two types, a thermal fusing image transfer type and a sublimation image transfer type. The sublimation image transfer type is advantageous over the thermal fusing image transfer type in that a halftone can be obtained without difficulty. This advantage can be obtained because a sublimable dye is in principle sublimated in the form of individually separated molecules in such an amount as to correspond to the amount of thermal energy applied thereto, for instance, from a thermal head, and transferred to the image receiving sheet. Therefore, the sublimation image transfer type is considered to be most suitable for full color printers.
The sublimation-type thermal image transfer recording method, however, has the shortcoming in that its running cost is high, because in this image transfer recording method, a yellow ink sheet, a magenta ink sheet, a cyan ink sheet, and if necessary, a black ink sheet, are employed in order to obtain a full color image, with selective application of thermal energy to each ink sheet, and discarded after the recording even though large unused portions remain on each ink sheet.
To eliminate this shortcoming, the Applicants have proposed a sublimation-type thermal image transfer recording medium which has a laminated structure as disclosed in Japanese Patent Application 63-62866. More specifically, the sublimation-type thermal image transfer recording medium comprises a dye-supply layer and a dye-transfer layer. The dye transferable performance of the dye-supply layer to the image receiving sheet is made greater than that of the dye-transfer layer, thereby avoiding the deterioration of the image density even when multiple printing recording is carried out. In other words, when a dye-supply layer and a dye-transfer layer are separately provided in the form of a single layer by coating with their specific formulations in the same deposition amount on each support sheet, each layer is superimposed on an image receiving sheet, and an equal amount of thermal energy is applied to each layer through the support, and there is established the relationship that the amount of the sublimable dye to be transferred from the dye-supply layer to the image receiving sheet is more than the amount of the sublimable dye to be transferred from the dye-transfer layer to the image receiving sheet, the multiple printing recording function by using the sublimation-type thermal image transfer recording medium can be improved.
Furthermore, a multiple printing recording method by which an ink sheet can used repeatedly has been studied not only from the viewpoint of the image transfer recording medium, but also from the view point of a method of bringing the ink sheet into close contact with the image receiving sheet.
Specifically, there has been proposed two methods, an equal-speed mode method and an n-times-speed mode method. In the former method, an ink sheet and an image receiving sheet are moved at the same speed when image printing is repeated. In the latter method, the running speed of the image receiving sheet is n (n&gt;1) times the running speed of the ink sheet when image printing is conducted, so that the ink sheet is shifted relative to the image receiving sheet in such a manner that a preceding portion of the ink sheet and the following portion thereof partly overlap with respect to the ink transfer therefrom in the course of the thermal printing. Therefore as a matter of course, the larger the value of "n", the larger the cost reduction in printing.
In the n-times-speed mode method, the ink is supplied at least from a newly used portion of the ink sheet in each printing, so that the variations of the amount of a residual ink in the ink sheet can be more minimized in comparison with the equal-speed mode method in which a used portion of the ink sheet is merely used repeatedly. Therefore, the n-times-speed mode method is advantageous over the equal-speed mode method with respect to the minimization of the amount of the residual ink in the ink sheet from the viewpoint of the recording history of the ink sheet as reported in the Journal of the Institute of Electronics and Communication Engineers, Vol. J70-C, No. 11, pages 1537-1544 (1987).
The multiple printing recording method such as the n-times-speed mode method, however, have the drawback that the sublimable dye which has been already transferred to an image receiving layer of the image receiving sheet is transferred back to the ink layer of the thermal image transfer recording medium. As a result, in some cases, the color of the image, when subsequently formed, becomes unclear, and a tailing phenomenon takes place at the edge of the image on the image receiving sheet.
The above-mentioned drawback in the sublimation-type thermal image transfer recording method stems from the thermal diffusion of the sublimable dye from the ink layer of the thermal image transfer recording medium to the image receiving layer, which are closely brought into pressure contact with each other by a thermal head and a platen roller.
When a secondary or tertiary color is formed on the image receiving layer by superimposing two or three dyes in the full color printing process, the dye which has been already transferred to the image receiving layer is transferred back to the ink layer of the thermal image transfer recording medium. This phenomenon is a so-called "reverse transfer". In the case of one-time printing, the thermal image transfer recording sheet in which the reverse transfer has occurred is discarded, so that the above-mentioned problem does not affect the image formation. In the case of multiple printing recording, however, the image reversely transferred to the ink layer of the thermal image transfer recording sheet is transferred again to the other position of the image receiving layer, so that the color turbidity and the tailing phenomenon at the edge of the image and caused, and the subsequent recordings are adversely affected.
Furthermore, the sublimation-type thermal image transfer recording medium has a problem in that an ink layer and an image receiving layer become fused when the thermal recording is conducted, because of the low heat-resistance of an organic binder agent contained in the ink layer on the support and because of the low heat-resistance of an organic binder agent in the image receiving layer.
In addition to the above, when the n-times-speed mode method is employed, the difference between the running speed of the ink layer and that of the image receiving layer generates a frictional force between the surface of the ink layer and that of the image receiving layer. Moreover, the ink layer and the image receiving layer are fused by the application of heat by a thermal head when the recording is conducted. As a result, the improper running and the sticking between the ink layer and the image receiving sheet will take place.
Therefore, for example, a silicone oil is generally contained in the image receiving layer to prevent the thermal fusion of the ink layer of the thermal image transfer recording medium and the image receiving layer. In this case, as the content of the silicone oil in the image receiving layer is increased, the preservability of the printed images deteriorates. On the other hand, in the case where a resin with a high melting point and high heat-resistance is employed as an organic binder agent for use in the ink layer of the thermal image transfer recording medium, images with high density cannot be obtained because of the poor diffusion of the dye from the ink layer to the image receiving layer.
To solve the above-mentioned problem, Japanese Laid-Open Patent Application 3-128287 has disclosed a sublimation-type thermal image transfer recording medium with an ink layer which comprises a hydrolyzed product of a silane coupling agent. This sublimation-type thermal image transfer recording medium, however, has not solved the above-mentioned problem sufficiently.
In view of the above-mentioned facts, the following can be said:
1) Excellent multi printing performance cannot be obtained by the thermosensitive image transfer recording media with the conventional dye layer structure even when the deposition amount is increased.
2) In the case of a thermosensitive image transfer recording medium in general use, a dye which is already present on the surface of the image receiving layer of an image receiving sheet prior to the succeeding super-imposition of dyes is transferred back to the recording medium. As a result, the problems of the formation of unclear colors, ghost images, and the tailing of images at the edge portions thereof will occur during the succeeding image printings.
3) The organic binder agent contained in the ink layer and the organic binder agent contained in the image receiving layer are fused by the heat from a thermal head during the image printing steps.
4) In the n-times-speed mode method, because there is a difference in the transportation speed between the ink layer and the image receiving layer, and heat is applied thereto during the image printing steps, the ink layer and the image receiving layer are fused unless the contacting surfaces of the ink layer and the image receiving layer have appropriate heat resistance and lubricity.